In scanning tunneling microscopy, a probe tip is brought close to a sample so that a space of approximately few nanometers (nm) may be left between them, in order that the electron cloud of the tip and the electron cloud on the sample surface be superimposed. Under this condition, a voltage is applied between the tip and the sample. The resulting current is called tunneling current. When the applied voltage is several mV to several V, the tunneling current is approximately 1 to 10 nanoAmperes (nA). The magnitude of the tunneling current is proportional to the distance between the sample and the tip. Thus, this distance can be determined quite accurately by measuring the magnitude of the tunneling current. If the position of the tip is known, the shape of the sample surface can be determined. If the position of the tip is so controlled that the tunneling current is kept constant, then the geometry of the sample surface can be similarly determined by tracing the position of the tip. The principle of the scanning tunneling microscopy is explained in U.S. Pat. No. 4,343,993.
In scanning tunneling microscopy where a specimen, for example an integrated circuit chip, is scanned, the tip placed along the z-axis is moved along both x- and y-axes to make a two-dimensional scan of a sample whose flat surface is placed in the xy-plane. Heretofore, the cubic type and the tube type of tip have been available as the chip scanner.
FIG. 1(a) is a perspective view of a chip scanning device of the cubic type. FIG. 1(b) is a side elevation of the device. Piezoelectric devices 1x and 1y are used for scans made along the x- and y-axes, respectively. A piezoelectric element 2 is employed to control the position of the front end of a probe tip 3 along the z-axis. In the operation of the scanning device of the cubic type, a voltage is applied between both ends of each piezoelectric element to produce an electric field. The piezoelectric element is then polarized and distorted to make scans along the x- and y-axes. When a scan is made along the x-axis, the piezoelectric element 1x is distorted as indicated by the broken line in FIG. 1(b). A large stress is applied to the yz-plane. As a result, cracks may take place. For this reason, it has been impossible for the device of the cubic type to make a scan over a relatively broad range. When the piezoelectric element used for the scan made along the x-axis is stretched or contracted, the position of the chip taken on the y-axis is also varied. Therefore, the scan of the chip involves distortion.
FIG. 2(a) is a perspective view of a scanning device of the tube type. FIG. 2(b) is a side elevation of the device. Referring to FIG. 2(a), piezoelectric elements 4x', and 4x are used for coarse movement and fine movement, respectively, made along the x-axis. Piezoelectric elements 4y', and 4y are employed for coarse movement and fine movement, respectively, made along the y-axis. A piezoelectric element 5 is used for controlling the position taken on the z-axis. The element 5 is bonded to the elements 4x, 4x', 4y, and 4y'. A probe tip 6 is mounted on the element 5. In the operation of the scanning device of the tube type, the piezoelectric element 4x', is distorted by elongation, while the piezoelectric element 4x is distorted by contraction, for example. Then, as shown in FIG. 2(b), the tip 6 is scanned along the x-axis. With this device, it has been impossible to scan the sample at a high speed, because flexure of the elements is utilized.